Trunked communication systems are known to comprise a plurality of communication units, a limited number of communication resources that are transceived via a predetermined number of repeaters, or base stations, and a communication resource allocator that allocates the limited number of communication resources among the plurality of communication units. The communication resources may comprise a TDM bus, a carrier frequency, a pair of carrier frequencies, or any RF transmission means. Of the communication resources, one is selected as the control channel which transceives operational data between the communication resource allocator and the communication units. The communication units, which may be portable radios and/or mobile radios, are arranged into talk groups, also called user groups, by commonality of use. For example, a user group may comprise communication units that are operated by a police department while another user group comprises communication units operated by a fire department.
In a multi-site trunked communication system, the communication resources of one communication site may communicate to any other communication resource in any other communication site via an audio switch. Quite often in a dispatch call, more than one communication unit tries to transmit audio at the same time. Some systems determine which audio should be sourced based on signal quality. Other communication systems limit the communication users in the system to operate only in one type of audio sourcing mode, with very little control on a user group basis to determine which communication units are allowed to communicate at any one time. Thus, if a system manager needs to interrupt a call to deliver an important announcement, the manager must wait until the current communications are completed, even when the current call may be unduly long. The system manager needs to more flexibly determine call sourcing in the user groups in the system. Therefore, a more flexible method of allocation of resources is desired.